There is found to be a need for apparatus which simulates the outdoor natural environment to some degree, but is adapted for performing carefully controlled experiments relating to plant physiology. Many such experiments are designed and conducted for obtaining data on methods to maximize production of cultivated plants in a given period of time and utilizing a given amount of space. Some cultivated plant experiments are accomplished outdoors in a natural setting with concomitant variations of sunlight, rain, soil and abundant space.
But the natural outdoor setting in which plants are ordinarily cultivated does not provide the requisite milieu to perform carefully controlled experiments. Rainfall, wind and humidity are uncontrollable and unpredictable in the out-of-doors. So experiments to determine optimum levels of plant temperature, humidity, air flow and exposure to light or to determine whether genetic and environmental factors are responsible for certain observed differences among plants must be done in an environmentally controlled chamber.
Most naturally illuminated plant growth chambers in the prior art were designed and built for use inside a greenhouse where the temperature is easily controlled. Other plant growth chambers in the prior art also provided for controlled lighting by inclusion of electric light sources of controlled intensity arranged around the plants in the growth chamber. But it has been found that some plant species grow abnormally under the fluorescent and other types of lamps commonly used in electrically lit chambers.
For example, U.S. Pat. No. 4,850,135, to DeMarco, describes an indoor plant growth apparatus provided with fluorescent lighting because sunlight is unpredictable. In the disclosure of that patent, the lighting is turned on and off with alternating periods of darkness, there are provisions for controlled temperature and air circulation, and vents are provided for escape of air from the apparatus. But this plant irradiation by vertically directed light is potentially disadvantageous, because the light intensity received by the lower leaves of given plant is much less than that received by the upper leaves and the lowermost leaves of the group will have substantially reduced photosynthesis and may die.
The use of daylight should overcome the problem of abnormal plant behavior which presumably results from the spectral composition of the light from fluorescent lamps. The main advantage of day-lit chambers over those which incorporated electric lighting is that they allow plants to be grown in light simulating nature and also of similar quality to that available to commercial greenhouse growers. Another feature of day-lit chambers is that the light photosynthetic photon flux density varies by season, daily and even from minute to minute. This can be disadvantageous when making measurements which integrate changes in plants or environmental factors over periods of time, but has the advantage that plants in the chambers experience the natural daily and seasonal trends.
Furthermore, many environmental growth chambers or cabinets in the prior art were constructed with a wood, aluminum or other metallic frame. The problem confronted here is that such assembly involves significant heat transfer to the frame members, considerable air leakage, and the opaque frame members allow significant variance in the light available within the chamber because of the shadowing thereby created.
Furthermore, some prior art growth chambers with electric lighting have the attendant problem, that the lighting be turned on and off in a regular cycle, in order to provide photostimulation of the plants in the chamber and, also alternate with periods of relative darkness. This necessary on and off cycling of the lighting tends to substantially raise both the initial cost and the cost of operation of the air temperature control means. The wide variations in heat output of the lamps when switched on and off necessitates a higher capacity air conditioning system, so as to provide adequate cooling when the full array of lamps is switched on, and it may also necessitate a higher capacity air heating system to provide adequate heating capacity when the lamp array is switched off in cold weather environments.